This invention relates generally to the field of laser devices, and more specifically to laser mirrors having a circulating water heat exchanger adjacent the mirror or as an integral component of the mirror structure to cool the mirror during laser operation.
Certain conventionally used laser mirrors, and particularly certain high energy laser (HEL) mirrors are fabricated of molybdenum and comprise thin-walled molybdenum tubing or other passageway defining a heat exchanger brazed to the mirror for the purpose of conducting water therethrough for cooling the mirror during laser operation. The cooling means may be conventionally brazed to the mirror support structure using a high strength braze. In operation of the HEL using typical mirrors constructed as just described, and using such conventional coolant as deionized water, serious corrosion problems have been encountered. Corrosion by general attack on the molybdenum in the heat exchanger by the deionized water used as coolant in conventional HEL mirror has been observed to proceed at the rate of 0.002 inch per year or more. Galvanic corrosion between the molybdenum and the braze material may accelerate the deterioration process, seriously affecting the physical integrity of the heat exchanger. Since laser mirror faceplates, and supporting structures, and the heat exchanger brazed thereto, are by design conventionally thin (approximately 0.020 inch or less) in order to exhibit good heat transfer characteristics, such mirrors may be rapidly destroyed by corrosion of the water.
Molybdenum HEL mirrors of the kind just described are frequently and conventionally used and cost many thousands of dollars. Exact cost depends on mirror size, materials, and design requirements and complexity of structural configuration. Further, by reason of the corrosion processes just described, to which the laser mirror may be subjected, its useful operational life may be unacceptably short. Despite numerous attempts to solve the problem of molybdenum mirror structural deterioration, the problem has heretofore persisted without satisfactory solution.
The invention described herein provides an improved laser mirror, such as the HEL type, having a water cooled heat exchanger comprising water conducting passageways that are tungsten coated, which provide superior resistance to the general corrosive process of the coolant water, and eliminates galvanic attack and stress corrosion cracking in the molybdenum structure of the mirror. The tungsten coating to the inside surface of the molybdenum channels may be provided by such as the chemical vapor deposition (CVD) process disclosed herein, although other coating methods may be applied, as might occur to one with skill in the field of chemical vapor deposition techniques. Therefore, other metal coatings, such as tantalum, as might be applied using a process analogous to that disclosed herein may result in a mirror having suitable corrosive resistance. The process disclosed herein provides a laser mirror having a continuous tungsten coating on the interior surfaces of the molybdenum heat exchanger components which prevents several forms of corrosion by the coolant water on the molybdenum structure. The physical and mechanical properties of CVD tungsten or tantalum materials are similar to the properties of molybdenum, and, therefore, the coating adds structural strength to the mirror without presenting problems of thermal conductivity or thermal expansion mismatch between coating and structure.
Therefore, providing a laser mirror having a protective coating on the molybdenum heat exchanger passageways greatly extends the mirror life by substantially eliminating the corrosion problem associated with using circulating water as coolant. Corrosion by general attack does not stop entirely since tungsten is corroded by water though at a much slower rate. The coating may be replenished periodically according to the methods described herein at low cost to extend mirror life indefinitely.
These and other objects of the present invention, as might occur to one with skill in the field of this invention, will become apparent as the detailed description proceeds.